This invention relates to bone prostheses and more particularly to a modular shoulder prosthetic device that comprises a head, a body, and a stem.
The design of humeral prostheses for replacement of the shoulder joint has evolved through the years in at least three different stages. First stage designs were relatively simple, usually consisting of a partial ball on a stem as exemplified in U.S. Pat. No. 3,694,820 issued to Scales et al., assigned to the National Research Development Corporation of London, England. Stage two designs were more complex due to their modular nature, which typically consisted of a more sophisticated stem structure that was interchangeable in size and joined to a separate head of a selected size. A typical stage two humeral prosthesis is described in U.S. Pat. No. 5,507,817 issued to Craig et al., assigned to the Kirschner Medical Corporation. Third stage designs, which are most recent and represent the state of the art, typically consist of a three component modular structure including a head, body, and stem. These designs provide a change in the inclination of the head with resection angles of 35 to 50 degrees from the axis of the stem and variable offset of the head relative to the body. An example of a third stage humeral prosthesis design is described in U.S. Pat. No. 5,358,526 issued to Alain Tonier, assigned to Etablissements Tonier of Saint-Ismier, France.
Several commonly used humeral prosthesis designs have been studied and the findings reported in a paper entitled, GEOMETRIC ANALYSIS OF COMMONLY USED PROSTHETIC SYSTEMS FOR PROXIMAL HUMERAL REPLACEMENT, by M. L. Pearl, M.D. and Sam Kurutz, BS, published in the Journal of Bone and Joint Surgery, May 1999@660. The findings of this study tended to show that, despite their increasing sophistication, the second stage humeral prosthesis designs did not replicate normal anatomy, thus, precluding restoration of the normal biomechanics of the glenohumeral joint.
The above study, and other biomechanical studies offer several reasons to expect deranged biomechanics after humeral replacement. These reasons include: superior and lateral shifting of the center of rotation of the glenohumeral joint by 15 or more millimeters from the original position after humeral replacement; a diminished arc of the articulated surface as compared with the anatomical joint; patient to patient variations for which there is insufficient accommodation in most prostheses; variations in the location of the axis of the humeral canal as reamed; and common surgical techniques for removing the anatomical humeral head limit the positioning of the prosthetic head because they remove the anatomical humeral head at a standard or predetermined angle prior to definition of the prosthetic stem canal position.
Hence, a humeral prosthesis is needed which substantially allows replication of the normal anatomy.
According to an aspect of the present invention, a humeral prosthetic device comprises a body; a stem for anchoring the device in a humeral canal of a humerus, the stem coupled to a first area of the body; and a head for replacing a natural humeral head of the humerus, the head coupled to a second area of the body and disposed at a selected angle relative to the stem. At least one of the body and stem controls the selected angle of the head relative to the stem.
According to another aspect of the present invention, a humeral prosthetic device comprises a body; a stem for anchoring the device in a humeral canal of a humerus, the stem coupled to a first area of the body and defining a center line for centering the stem in the humeral canal; and a head for replacing a natural humeral head of the humerus, the head coupled to a second area of the body and disposed at a selected angle relative to the stem; the stem including an extension for coupling the stem to the body, the extension disposed on an axis parallel to and offset from the center line for providing a lateral offset of the body relative to the stem upon reversal of the stem in the humeral canal.
According to a further aspect of the present invention, a humeral prosthetic device comprises an insert which coacts with a body and a head of the device to provide superior, inferior, anterior, and posterior positioning of the head relative to the body.